Emergency locating transmitters (ELTs) are required by federal law on all aircraft travelling more than twenty-five miles from an airport, and emergency position indicating radio beacons (EPIRBs) are required on certain classes of marine vessels. ELTs and EPIRBs are essentially the same device. They differ only in the manner in which they are activated. ELTs are activated by gravitational forces (impact) while EPIRBs are activated by water contact. Both, however, may be manually activated.
Prior art ELTs and EPIRBs transmit a distress waveform on 121.5 MHz and 243 MHz to alert emergency frequency monitors that a distress incident has occurred. The distress waveform transmitted by these devices consists of an amplitude modulated carrier signal in which the modulating signal is an audio frequency sweeping downward over a range of not less than 700 Hz, within the range of 1,600 Hz to 300 Hz, and at a sweep rate varying between 2 Hz and 4 Hz. These characteristics are required by federal law, so that the transmitted distress waveform, which can be demodulated by a receiver to a siren-like sound, may easily be recognized by individuals monitoring on 121.5 MHz and 243 MHz, who can then alert search and rescue (SAR) personnel to search for the location of the source of the distress transmission and initiate rescue operations. The distress waveform, however, does not contain information other than that an ELT or EPIRB is transmitting it. Accordingly, SAR personnel receive no advance information on whether they are searching for an aircraft or a marine vessel. Additional information, such as information which identifies a specific class of transmitter and the identity of the specific vehicle in distress, would greatly aid SAR personnel and allow for more efficient use of SAR resources. Transmitter class information is important because once a receiver identifies a signal as one containing vehicle identity information, it can then extract that information. Vehicle identity information is important because it is an indication of the maximum number of people which need to be rescued. It may also give an indication of what type of rescue operations may be required.